Sacral tether anchor and methods of use

ABSTRACT

A system for restricting flexion of a spinal segment in a patient comprises a constraint device having a tether structure and a compliance member coupled with the tether structure. The tether structure is adapted to be coupled with a superior spinous process and a sacrum. The system also includes an anchor member that is anchored to the sacrum. The anchor member has an attachment feature that is adapted to couple with the constraint device.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 17/020,463 (Attorney Docket No. 48626-712.306), filed Sep. 14,2020, which is a continuation of U.S. patent application Ser. No.15/863,716 (Attorney Docket No. 48626-712.305), filed Jan. 5, 2018,which is a continuation of U.S. patent application Ser. No. 15/279,046(Attorney Docket No. 48626-712.304) filed Sep. 28, 2016 which is acontinuation of U.S. patent application Ser. No. 14/598,932 (AttorneyDocket No. 48626-712.303) filed Jan. 16, 2015, which is a continuationof U.S. patent application Ser. No. 13/963,786 (Attorney Docket No.48626-712.302) filed Aug. 9, 2013, which is a continuation of U.S.patent application Ser. No. 13/193,441 (Attorney Docket No.48626-712.301), filed Jul. 28, 2011, which is a continuation of PCTApplication No. PCT/US2010/022767 (Attorney Docket No. 48626-714.601),filed Feb. 1, 2010, which claims the benefit of U.S. Provisional PatentApplication No. 61/149,224 (Attorney Docket No. 48626-714.101) filedFeb. 2, 2009; the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention generally relates to medical methods andapparatus. More particularly, the present invention relates to methodsand apparatus used to couple a prosthesis to a spinal segment. Often, aportion of the prosthesis may be coupled to the sacrum. The methods andapparatus disclosed herein may be used during orthopedic internalfixation procedures. This includes but is not limited to treatment ofpatients having back pain or other spinal conditions.

A major source of chronic low back pain is discogenic pain, also knownas internal disc disruption. Patients suffering from discogenic paintend to be young, otherwise healthy individuals who present with painlocalized to the back. Discogenic pain usually occurs at the discslocated at the L4-L5 or L5-S1 junctions of the spine. Pain tends to beexacerbated when patients put their lumbar spines into flexion (i.e. bysitting or bending forward) and relieved when they put their lumbarspines into extension (i.e. by standing or arching backwards). Flexionand extension are known to change the mechanical loading pattern of alumbar segment. When the segment is in extension, the axial loads borneby the segment are shared by the disc and facet joints (approximately30% of the load is borne by the facet joints). In flexion, the segmentalload is borne almost entirely by the disc. Furthermore, the nucleusshifts posteriorly, changing the loads on the posterior portion of theannulus (which is innervated), likely causing its fibers to be subjectto tension and shear forces. Segmental flexion, then, increases both theloads borne by the disc and causes them to be borne in a more painfulway. Discogenic pain can be quite disabling, and for some patients, candramatically affect their ability to work and otherwise enjoy theirlives.

Pain experienced by patients with discogenic low back pain can bethought of as flexion instability, and is related to flexion instabilitymanifested in other conditions. The most prevalent of these isspondylolisthesis, a spinal condition in which abnormal segmentaltranslation is exacerbated by segmental flexion. The methods and devicesdescribed herein should as such also be useful for these other spinaldisorders or treatments associated with segmental flexion, for which theprevention or control of spinal segmental flexion is desired. Anotherapplication for which the methods and devices described herein may beused is in conjunction with a spinal fusion, in order to restrictmotion, promote healing, and relieve pain post-operatively.Alternatively, the methods and devices described should also be usefulin conjunction with other treatments of the anterior column of thespine, including kyphoplasty, total disc replacement, nucleusaugmentation and annular repair.

Patients with discogenic pain accommodate their syndrome by avoidingpositions such as sitting, which cause their painful segment to go intoflexion, preferring positions such as standing, which maintain theirpainful segment in extension. One approach to reducing discogenic paininvolves the use of a lumbar support pillow often seen attached tooffice chairs. Biomechanically, the attempted effect of the ubiquitouslumbar support pillow is also to maintain the painful lumbar segment inthe less painful extension position.

Current treatment alternatives for patients diagnosed with chronicdiscogenic pain are quite limited. Many patients follow a conservativetreatment path, such as physical therapy, massage, anti-inflammatory andanalgesic medications, muscle relaxants, and epidural steroidinjections, but typically continue to suffer with a significant degreeof pain. Other patients elect to undergo spinal fusion surgery, whichcommonly requires discectomy (removal of the disk) together with fusionof adjacent vertebra. Fusion may or may not also include instrumentationof the affected spinal segment including, for example, pedicle screwsand stabilization rods. Fusion is not lightly recommended for discogenicpain because it is irreversible, costly, associated with high morbidity,and has questionable effectiveness. Despite its drawbacks, however,spinal fusion for discogenic pain remains common due to the lack ofviable alternatives.

An alternative method, that is not commonly used in practice, but hasbeen approved for use by the United States Food and Drug Administration(FDA), is the application of bone cerclage devices which can encirclethe spinous processes or other vertebral elements and thereby create arestraint to motion. Physicians typically apply a tension or elongationto the devices so that they apply a constant and high force on theanatomy, thereby fixing the segment in one position and allowingeffectively no motion. The lack of motion allowed after the applicationof such devices is thought useful to improve the likelihood of fusionperformed concomitantly; if the fusion does not take, these devices willfail through breakage of the device or of the spinous process to whichthe device is attached. These devices are designed for staticapplications and are not designed to allow for dynamic elasticresistance to flexion across a range of motion. The purpose of bonecerclage devices and other techniques described above is to almostcompletely restrict measurable motion of the vertebral segment ofinterest. This loss of motion at a given segment gives rise to abnormalloading and motion at adjacent segments, which can lead eventually toadjacent segment morbidity.

An alternative solution that avoids some of the challenges associatedwith cerclage devices involves the use of an elastic structure, such astether structures, coupled to the spinal segment. The elastic structurecan relieve pain by increasing passive resistance to flexion while oftenallowing substantially unrestricted spinal extension. This mimics themechanical effect of postural accommodations that patients already useto provide relief.

Spinal implants using tether structures are currently commerciallyavailable. One such implant couples adjacent vertebrae via theirpedicles. This implant includes spacers, tethers and pedicle screws. Toinstall the implant, muscles are retracted to a wide extent to exposethe pedicles, and selected portions of the disc and vertebrae bone maybe removed. Implants are then placed to couple two adjacent pedicles oneach side of the spine. The pedicle screws secure the implants in place.The tether is clamped to the pedicle screws with set-screws, and limitsthe extension/flexion movements of the vertebrae of interest, as well aslimiting other motions such as axial compression, later bending, androtation. Because significant tissue is displaced and/or removed andbecause of screw placement into the pedicles, the implant andaccompanying surgical methods are highly invasive and the implant isoften irreversibly implanted. There is also an accompanying significantchance of nerve root damage. Additionally, the tip of the set-screwclamps the tethers, and this may result in abrasion of the tethers alongwith generation of particulate wear debris.

Other implants employing tether structures couple adjacent vertebrae viatheir processes instead. These implants include a tether and a spacer.To install the implant, the supraspinous ligament is temporarily liftedand displaced. The interspinous ligament between the two adjacentvertebrae of interest is then permanently removed and the spacer isinserted in the interspinous space. The tether is then wrapped aroundthe processes of the two adjacent vertebrae, through adjacentinterspinous ligaments, and then mechanically secured in place by thespacer or also by a separate component fastened to the spacer. Thesupraspinous ligament is then restored back to its original position.Such implants and accompanying surgical methods are not withoutdisadvantages. These implants may subject the spinous processes tofrequent, high loads during everyday activities, sometimes causing thespinous processes to break or erode. Furthermore, the spacer may put apatient into segmental kyphosis, potentially leading to long-termclinical problems associated with lack of sagittal balance. The processof securing the tethers is often a very complicated maneuver for asurgeon to perform, making the surgery much more invasive. And, aspreviously mentioned, the removal of the interspinous ligament ispermanent. As such, the application of the device is not reversible.

More recently, less invasive spinal implants have been introduced. Likethe aforementioned implant, these spinal implants are placed over one ormore pairs of spinous processes and provide an elastic restraint to thespreading apart of the spinous processes occurring during flexion.However, extension-limiting spacers are not used and interspinousligaments need not be permanently removed. As such, these implants areless invasive and may be reversibly implanted. The implants typicallyinclude a tether structure and a securing mechanism for the tether. Thetether may be made from a flexible polymeric textile such as wovenpolyester (PET) or polyethylene (e.g. ultra high molecular weightpolyethylene, UHMWPE); multi-strand cable, or other flexible structure.The tether is wrapped around the processes of adjacent vertebrae andthen secured by the securing mechanism. Securing mechanisms aredescribed in greater detail below.

While the constraint devices described above appear to be promising, insome situations, attachment of the device to a spinous process can bechallenging. For example, if the constraint device is attached to asmall spinous process that does not protrude enough or has geometryunsuitable to engage a tether, such as steeply sloping surfaces, theconstraint device could migrate or disengage from the spinous processafter implantation. Furthermore, it may be necessary to couple theconstraint device with an upper spinous process disposed on a superiorvertebra and an inferior spinous process, crest or tubercle disposed onthe sacrum (e.g. for implantation at the L5-S1 level). Often, thespinous processes in these regions are small and do not protrudesufficiently to provide an adequate attachment point for the constraintdevice. In other cases where a spinous process of sufficient size ispresent, the surfaces may slope such that the constraint device wouldtend to migrate or slip off the process. Therefore, it would bedesirable to provide apparatus and methods that facilitate attachment ofthe constraint device to a small or sloping spinous processes, sacralcrest or tubercle, in particular one disposed on the sacrum or directlyto the sacrum. Moreover, it would also be desirable if such devices andmethods were easy to use and minimally invasive to the patient.

2. Description of the Background Art

Patents and published applications of interest include: U.S. Pat. Nos.3,648,691; 4,643,178; 4,743,260; 4,966,600; 5,011,494; 5,092,866;5,116,340; 5,180,393; 5,282,863; 5,395,374; 5,415,658; 5,415,661;5,449,361; 5,456,722; 5,462,542; 5,496,318; 5,540,698; 5,562,737;5,609,634; 5,628,756; 5,645,599; 5,725,582; 5,902,305; Re. 36,221;5,928,232; 5,935,133; 5,964,769; 5,989,256; 6,053,921; 6,248,106;6,312,431; 6,364,883; 6,378,289; 6,391,030; 6,468,309; 6,436,099;6,451,019; 6,582,433; 6,605,091; 6,626,944; 6,629,975; 6,652,527;6,652,585; 6,656,185; 6,669,729; 6,682,533; 6,689,140; 6,712,819;6,689,168; 6,695,852; 6,716,245; 6,761,720; 6,835,205; 7,029,475;7,163,558; Published U.S. Patent Application Nos. 2002/0151978;2004/0024458; 2004/0106995; 2004/0116927; 2004/0117017; 2004/0127989;2004/0172132; 2004/0243239; 2005/0033435; 2005/0049708; 2005/0192581;2005/0216017; 2006/0069447; 2006/0136060; 2006/0240533; 2007/0213829;2007/0233096; 2008/0009866; 2008/0108993; 2008/0177264; 2008/0108993;2008/0262549; Published PCT Application Nos. WO 01/28442 A1; WO 02/03882A2; WO 02/051326 A1; WO 02/071960 A1; WO 03/045262 A1; WO2004/052246 A1;WO 2004/073532 A1; WO2008/051806; WO2008/051423; WO2008/051801;WO2008/051802; and Published Foreign Application Nos. EP0322334 A1; andFR 2 681 525 A1. The mechanical properties of flexible constraintsapplied to spinal segments are described in Papp et al. (1997) Spine22:151-155; Dickman et al. (1997) Spine 22:596-604; and Garner et al.(2002) Eur. Spine J. S186-S191; Al Baz et al. (1995) Spine 20, No. 11,1241-1244; Heller, (1997) Arch. Orthopedic and Trauma Surgery, 117, No.1-2:96-99; Leahy et al. (2000) Proc. Inst. Mech. Eng. Part H: J. Eng.Med. 214, No. 5: 489-495; Minns et al., (1997) Spine 22 No.16:1819-1825; Miyasaka et al. (2000) Spine 25, No. 6: 732-737; Shepherdet al. (2000) Spine 25, No. 3: 319-323; Shepherd (2001) Medical Eng.Phys. 23, No. 2: 135-141; and Voydeville et al (1992) Orthop Traumatol2:259-264.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to methods and apparatus used to couple aprosthesis to a spinal segment and adjust the prosthesis duringorthopedic internal fixation procedures. This includes but is notlimited to coupling a constraint device between a superior spinousprocess and the sacrum during treatment of patients having spinal pain,instability or other spinal conditions.

In a first aspect of the present invention, a system for restrictingflexion of a spinal segment in a patient comprises a constraint devicehaving a tether structure and a compliance member coupled with thetether structure. The tether structure comprises an upper portionadapted to be coupled with a superior spinous process and a lowerportion adapted to be coupled with a sacrum. The constraint deviceprovides a force resistant to flexion of the spinal segment. The systemalso has an anchor member comprising an attachment member and a couplingmember. The attachment member is adapted to attach the anchor member tothe sacrum, and the coupling member is adapted to couple the tetherstructure with the anchor member.

The system may further comprise a second compliance member coupled withthe tether structure. The compliance members may be disposedsubstantially parallel to one another. They may be disposed on oppositesides of a midline of the spinal segment. The upper portion of thetether structure may comprise first and second free ends. Each of thefree ends may be coupled with one of the compliance members. The lowerportion of the tether structure may comprise a loop which may be coupledto the coupling member.

The anchor member may comprise an elongate bar, and the bar may bedisposed across the midline of the spinal segment. The bar may comprisea through hole that is adapted to receive the attachment member, whichattaches the bar to the sacrum. The bar may have an adjustable length.The anchor member may comprise a plate in engagement with the sacrum.The anchor may be disposed in a recessed region of the sacrum so that anouter surface of the anchor member is substantially flush with an outersurface of the sacrum. The anchor member may comprise a staple, or acable.

The attachment member may comprise a screw threadably engaged with thesacrum. The screw may be axially threaded into engagement with a crestof the sacrum in a generally caudal direction. The attachment member maycomprise a nail, a clip, or a hook that is adapted to couple with thesacrum.

The coupling member may comprise one of a hook, an eyelet, a slot, and apin. The coupling member may comprise a channel disposed in a portion ofthe anchor member. The channel may be adapted to receive the tetherstructure therethrough. The system may further comprise a liner elementdisposed in the channel. The coupling member may comprise a protuberanceadapted to engage the tether structure, or the coupling member maycomprise a pin that is adapted to engage the tether structure. Thecoupling member may comprise a gate mechanism having an open positionand a closed position. In the open position, the coupling member mayreceive the tether structure and in the closed position, the tetherstructure may be captured by the coupling member.

The system may further comprise a bone removal tool. The bone removaltool may be configured to remove bone from the sacrum therebyfacilitating attachment of the anchor member thereto. The bone removaltool may comprise a rongeur tool, or a drill. The bone removal tool maycomprise an elongate shaft, a handle coupled with the shaft, and acutting element having a crescent shaped cross-section. The cuttingelement may extend laterally from the shaft, and it may be adapted toremove bone from the sacrum. The bone removal tool may comprise aknurled cutting surface adapted to remove bone from the sacrum.

The anchor member may comprise a surface that is adapted to promoteosseointegration of the anchor with the sacrum. The surface may comprisehydroxyapatite or titanium.

In another aspect of the present invention, a system for restrictingflexion of a spinal segment in a patient comprises a constraint devicecomprising a tether structure and a compliance member coupled with thetether structure. The tether structure comprises an upper portion and alower portion. The upper portion is adapted to be coupled with asuperior spinous process and the lower portion is adapted to be coupledwith a coccyx. The constraint device provides a force resistant toflexion of the spinal segment. The lower portion may comprise a loop andthe loop may be disposed around the coccyx. The lower portion may belonger than the upper portion.

In still another aspect of the present invention, a method forrestricting flexion in a spinal segment comprises providing a constraintdevice having a tether structure, a compliance member coupled with thetether structure, and an anchor member. The tether structure comprisesan upper portion and a lower portion. The upper portion of the tetherstructure is engaged with a superior spinous process, and the anchormember is attached to a sacrum. The lower portion of the tetherstructure is coupled with the anchor member.

The method may further comprise resisting flexion of the spinal segment.The step of engaging the upper portion of the tether structure maycomprise disposing the upper portion of the tether structure over asuperior surface of the spinous process.

The step of attaching the anchor member may comprise removing bone fromthe sacrum. Attaching the anchor member may comprise forming a recessedregion in the sacrum. Attaching the anchor member may comprise creatinga notched region or a channel in a crest of the sacrum. The attachingstep may also comprise adjusting length of the anchor member. Attachingthe anchor member may comprise threadably engaging a fastener with thesacrum. The fastener may be threadably engaged in a generally caudaldirection so as to avoid penetration of the fastener into a spinal canalor into an anterior cortex of the sacrum. Attaching may comprisestapling or nailing the anchor member to the sacrum, or clipping aportion of the anchor member to a sacral crest or a spinous process ofthe sacrum. Attaching the anchor member may comprise hooking a portionof the anchor member with a neural foramen of the sacrum, or with alateral edge of the sacrum.

The anchor member may comprise a through hole, and the step of couplingthe lower portion of the tether structure comprises advancing the lowerportion of the tether structure through the through hole. The anchormember may comprise an elongate channel, and the step of coupling thelower portion of the tether structure may comprise advancing the lowerportion of the tether structure through the elongate channel. The stepof coupling the lower portion of the tether structure may comprise atleast partially encircling the lower portion of the tether structurearound a portion of the anchor member or the coupling member. Couplingthe lower portion of the tether structure may comprise opening a gate inthe anchor member thereby permitting the anchor member to receive thetether structure. The gate may also be closed, thereby capturing thetether structure. The method may further comprise promotingosseointegration of the anchor member with the sacrum, or adjustinglength or tension in the constraint device.

In another aspect of the present invention, a method for restrictingflexion of a spinal segment in a patient comprises providing aconstraint device having a tether structure that has an upper portionand a lower portion. The upper portion of the tether structure isengaged with a superior spinous process, and bone is removed from thesacrum in order to form an attachment region in the bone. The lowerportion of the tether structure is coupled with the attachment region.

The method may further comprise resisting flexion of the spinal segment.The constraint device may further comprise a compliance member coupledwith the tether structure. The step of engaging the upper portion of thetether structure may comprise disposing the upper portion of the tetherstructure over a superior surface of the spinous process.

The step of removing bone may comprise creating a channel extendingthrough a crest of the sacrum. The channel may be lined with a linerelement. Removing bone may comprise notching the sacrum, or it maycomprise one of cutting, grinding, drilling, filing, rasping, sawing,and abrading the sacrum. Removing bone may comprise removing the bonewith one of a rongeur tool, a cutting tool, a file, a rasp, a saw, and adrill.

The step of coupling the lower portion of the tether structure maycomprise advancing the lower portion through a channel in the sacrum.The method may further comprise preserving ligaments or other anatomicalstructures disposed along a midline of the spinal segment.

In still another aspect of the present invention, a method forrestricting flexion of a spinal segment in a patient comprises providinga constraint device having a tether structure, and a compliance membercoupled with the tether structure. The tether structure comprises anupper portion and a lower portion. The upper portion of the tetherstructure is engaged with a superior spinous process, and the lowerportion of the tether structure is coupled with a coccyx. Coupling thelower portion may comprise encircling at least a portion of the coccyxwith the lower portion of the tether structure. The lower portion of thetether structure may comprise a loop, and the step of coupling the lowerportion may comprise placing the loop around the coccyx.

These and other embodiments are described in further detail in thefollowing description related to the appended drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram illustrating the lumbar and sacralregions of the spine.

FIG. 1B a schematic illustration showing a portion of the lumbar regionof the spine taken along a sagittal plane.

FIG. 2A illustrates a spinal implant of the type described in U.S.Patent Publication No. 2005/0216017A1.

FIG. 2B illustrates the sacrum.

FIGS. 3A-3B illustrate the use of an undercut in the sacrum foranchoring a constraint device.

FIGS. 4A-4C illustrate the use of an anchor member for securing aconstraint device to the sacrum.

FIGS. 5A-5F illustrate use of a notched region in the sacrum foranchoring a constraint device.

FIG. 6 illustrates an exemplary embodiment of a notching tool.

FIGS. 7A-7B illustrates use of an aperture in the sacrum for attachmentof a constraint device.

FIGS. 8A-8B illustrate an exemplary embodiment of a punch tool.

FIGS. 8C-8F illustrate additional embodiments of bone removal tools.

FIGS. 9A-9E illustrate various embodiments of a crossbar.

FIG. 10 illustrates use of a screw as an anchor.

FIGS. 11A-11D illustrate an anchor coupled with the sacrum.

FIG. 12 illustrates use of a screw or pin as an anchor.

FIG. 13 illustrates another embodiment of a screw or pin as the anchor.

FIGS. 14A-14B illustrate still other embodiments of a screw or pin asthe anchor.

FIGS. 15A-15D illustrate additional embodiments of a screw or pin as theanchor.

FIGS. 16A-16F illustrate various embodiments of pins or posts that maybe used as the anchor.

FIGS. 17A-17B illustrate two exemplary embodiments of screws that may beused as the anchor.

FIGS. 18A-18B illustrate a liner.

FIG. 19 illustrates another embodiment of a liner.

FIGS. 20A-20B illustrate another embodiment of a liner.

FIG. 21 illustrates a reinforcement device.

FIG. 22 illustrates the use of a retaining pin.

FIGS. 23A-23B illustrate retaining bar embodiments.

FIG. 24A-24C illustrate retaining clips.

FIGS. 25A-25E illustrate additional retaining clip embodiments.

FIGS. 26A-26D illustrate still other retaining clip embodiments.

FIG. 27 illustrates a gate mechanism.

FIGS. 28A-28C illustrate a retention plate.

FIG. 29 illustrates a hook embodiment.

FIGS. 30A-30D illustrate the anatomy of a neural foramen and variousembodiments of neural foramina hooks.

FIG. 31 illustrates additional embodiments of sacral hooks.

FIG. 32 illustrates attachment of a constraint device with the coccyx.

FIGS. 33A-33B illustrate several embodiments of constraint devices whichmay be attached to the coccyx.

FIGS. 34A-34B illustrate surgical methods of coupling a constraintdevice with the coccyx.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A is a schematic diagram illustrating the lumbar region of thespine including the spinous processes (SP), facet joints (FJ), lamina(L), transverse processes (TP), and sacrum (S). FIG. 1B is a schematicillustration showing a portion of the lumbar region of the spine takenalong a sagittal plane and is useful for defining the terms “neutralposition,” “flexion,” and “extension” that are often used in thisdisclosure.

As used herein, “neutral position” refers to the position in which thepatient's spine rests in a relaxed standing position. The “neutralposition” will vary from patient to patient. Usually, such a neutralposition will be characterized by a slight curvature or lordosis of thelumbar spine where the spine has a slight anterior convexity and slightposterior concavity. In some cases, the presence of the constraint ofthe present invention may modify the neutral position, e.g. the devicemay apply an initial force which defines a “new” neutral position havingsome extension of the untreated spine. As such, the use of the term“neutral position” is to be taken in context of the presence or absenceof the device. As used herein, “neutral position of the spinal segment”refers to the position of a spinal segment when the spine is in theneutral position.

Furthermore, as used herein, “flexion” refers to the motion betweenadjacent vertebrae in a spinal segment as the patient bends forward.Referring to FIG. 1B, as a patient bends forward from the neutralposition of the spine, i.e. to the right relative to a curved axis A,the distance between individual vertebrae L on the anterior sidedecreases so that the anterior portion of the intervertebral disks D arecompressed. In contrast, the individual spinous processes SP on theposterior side move apart in the direction indicated by arrow B. Flexionthus refers to the relative movement between adjacent vertebrae as thepatient bends forward from the neutral position illustrated in FIG. 1B.

Additionally, as used herein, “extension” refers to the motion of theindividual vertebrae L as the patient bends backward and the spineextends from the neutral position illustrated in FIG. 1B. As the patientbends backward, the anterior ends of the individual vertebrae will moveapart. The individual spinous processes SP on adjacent vertebrae willmove closer together in a direction opposite to that indicated by arrowB.

FIG. 2A shows a spinal implant of the type described in related U.S.Patent Publication No. 2005/0216017 A1 (now U.S. Pat. No. 7,458,981),the entire contents of which are incorporated herein by reference. Asillustrated in FIG. 2A, an implant 10 typically comprises a tetherstructure having an upper strap component 12 and a lower strap component14 joined by a pair of compliance elements 16. The upper strap 12 isshown disposed over the top of the spinous process SP4 of L4 while thelower strap 14 is shown extending over the bottom of the spinous processSP5 of L5. The compliance element 16 will typically include an internalelement, such as a spring or rubber block, which is attached to thestraps 12 and 14 in such a way that the straps may be “elastically” or“compliantly” pulled apart as the spinous processes SP4 and SP5 moveapart during flexion. In this way, the implant provides an elastictension on the spinous processes which provides a force that resistsflexion. The force increases as the processes move further apart.Usually, the straps themselves will be essentially non-compliant so thatthe degree of elasticity or compliance may be controlled and providedsolely by the compliance elements 16. Additional details on constraintdevices is disclosed in U.S. patent application Ser. No. 12/106,103(Attorney Docket No. 026398-000410US) which is incorporated herein byreference. Any of these constraint devices may be used with theanchoring methods and apparatus disclosed herein. In other embodiments,the constraint device may be a tether structure with or without acompliance member or element. The tether structure may be elastic.

The flexion-limiting device may also be attached to an upper spinousprocess disposed on a vertebra and the sacrum in order to limit flexionbetween the sacrum and the upper vertebra. However, the sacrum often hasspinous processes that are very short and rounded; these are alsoreferred to as spinous tubercles. The sacrum may only have a low crestalong the dorsal midline. These regions are often insufficient for atether to loop around since the bone protruding from the sacrum may betoo short and the oblique angle of the sacral surfaces around which thedevice would loop may permit the device to migrate dorsally, andpotentially slide off. FIG. 2B illustrates the spinous processes SP onthe sacrum.

Referring now to FIG. 3A, a top view of the sacrum shows how an undercut302 may be formed by carving, cutting, sawing, grinding or othersuitable means, into the sacrum S. Undercut 302 in this exemplaryembodiment is a T-shaped slot that is adapted to receive a pin or otheranchor. FIG. 3B is a cross-sectional sideview of the sacrum and undercut302. In FIG. 3B, a constraint device having a tether 304 is anchored tothe sacrum S by engaging the tether 304 with the undercut 302. A freeend 310 of tether 304 is wrapped around a pin 306 and the free end 310is then fastened to the tether using methods well known in the art suchas stitching, thermal welding, suturing, bonding, riveting, etc.Alternatively, the free end 310 may return to an attachment point on theconstraint device. Tether 304 and pin 306 are then slidably received inthe T-shaped slot 302, with the tether 304 exiting the central portionof slot 302. The tether 304 and pin 306 are advanced in the cranialdirection until the pin 306 bottoms out in the slot thereby capturingthe pin 306 and tether 304. An optional fastener 308, such as a screw orpin may be advanced through the sacrum and tether/pin in order tofurther secure the device into the slot 302. In this exemplaryembodiment a T-shaped slot is used, however one will appreciate thatother geometries may also be used. The remainder of the constraintdevice may be secured to a superior portion of the affected spinalsegment such as a spinous process, pedicle, transverse process, etc.Tools which may be used to create the undercut are described below.

FIGS. 4A-4C illustrate the use of a washer to attach a constraint deviceto the sacrum S. In FIG. 4A a circular washer 402 has a central aperture404 and a tab 406 for engagement with the constraint device. Tab 406 hasan oblong aperture 408 for receiving a tether of the constraint device.A fastener such as a screw may be received in aperture 404 andthreadably engaged with the sacrum S in order to secure the washer 402to the bone. In FIG. 4B, optional recessed regions 410 are formed oneither side of the spinal segment midline so that the outer surface ofwasher 402 will be substantially flush with the outer surface of thesacrum. Recessed regions 410 may be formed by cutting, carving,grinding, or other means known in the art and these recessed regions arealso useful in helping to distribute loading more evenly along theinterface between the anchor and the bone, thereby reducing thelikelihood of mechanical failure. In this embodiment coatings such ashydroxyapatite or titanium may be deposited over an outer surface of thewasher in order to improve fixation through osseointegration. Surfacefeatures such as beading may also be used to enhance integration of theanchor with the bone. These coatings and surface features may be used inany of the implants described in this disclosure where bone ingrowth isdesirable. While a circular washer is disclosed in this embodiment, oneof ordinary skill in the art will appreciate that many other geometriesmay be substituted, some of which may provide even better fixation ofthe anchor to the sacrum. FIG. 4C is a cross-section of the sacrum Sshowing the washer 402 disposed in a recess 410 formed in the sacrum S.A fastener 416, here a screw, is disposed through aperture 404 andthreadably engaged with the sacrum S. Tether 412 has a free end 414 thatis fed through aperture 408 and then fixed to itself using any of themeans previously disclosed with reference to FIGS. 3A-3B. Exemplaryscrews which may be used include, but are not limited to those disclosedin FIGS. 17A-17B described in greater detail below. Tools which may beused to create the recessed region are also described below.

FIGS. 5A-5F illustrate how a notch may be created in the sacrum S andused to anchor a tether or a strap of a constraint device. The notch maybe created in the sacral crest, a spinous process or a tubercle. In FIG.5A a notched region 502 is formed in a crest of the sacrum S. The notch502 lengthens the surface of engagement available for a strap if aspinous process is present but insufficient to restrain the strap, orthe notch creates a raised region 504 that is similar to a naturallyoccurring spinous process. FIG. 5B is a side view showing notch 502 cutinto the sacrum S, thereby forming the raised region 504. A tether maythen be secured to the sacrum S by looping the tether directly throughthe notch 502 and thus no additional hardware is required. In someembodiments it is desirable to place hardware into the notch 502 inorder to protect the tether/bone interface. In FIG. 5C, a plate orsheave 508 is fixed in the notch 502. The plate 508 in this embodimentis secured to the notch with a screw 506. FIG. 5D illustrates anexemplary embodiment of a plate 508 having a curved region 510 thatforms a hook for engaging a tether structure. The plate or sheave 508may be fabricated from any number of materials including metals such asstainless steel or titanium, ceramics, polymers or other biocompatiblematerials. One of skill in the art will appreciate that a variety ofgeometries may be used as a protective liner element such as the plate,sheave, or other liners described below. FIG. 5E shows how theconstraint device is coupled with the notch in the sacrum S, with orwithout the protective sheave. In FIG. 5E, a constraint device 512 has acompliance member 514 and a tether structure 516. The tether 516 ispassed through the notch 502, thereby securing the constraint device tothe sacrum. An upper portion of the constraint device 512 may be coupledwith a superior spinous process (not illustrated).

FIG. 5F illustrates an alternative embodiment of a notched sacrum usedto retain the tether. In FIG. 5F a notch is created in the sacrum havinga keyway which includes a narrow slotted region 522 that opens up intoan enlarged aperture or slotted region 520. The narrow slotted region522 is sized to receive the tether 516. In this exemplary embodiment,the tether is a rectangular shaped strap having upper and lower planarsurfaces that represent the width of the strap and the tether also hasfront and back side walls generally perpendicular to the upper and lowerplanar surfaces that represent the strap thickness. Thus, when thetether 516 is oriented in a desired position, the tether may be advancedinto the slotted region. In this embodiment, the aperture can receivethe tether when the tether is advanced such that the tether leading edgeis one of the side walls. This embodiment is also advantageous becausewhen the tether structure is in tension, forces are distributed evenlyalong the surface of the enlarged slotted region 520. The enlargedaperture 520 may be drilled or otherwise cut in the sacrum and thenarrow slotted region 522 may be cut with a saw or other bone removingtools. Additional details on tether structures and compliance memberswhich may be applied to the embodiments disclosed in FIGS. 5A-5F or anyof the tether structures or compliance members disclosed herein, aredisclosed in U.S. patent application Ser. No. 12/106,103 (AttorneyDocket No. 0216398-000410US), entitled “Methods and Devices forControlled Flexion Restriction of Spinal Segments,” filed Apr. 18, 2008,the entire contents of which are incorporated herein by reference.

Various tools and fixtures may be used to create the notch in FIGS.5A-5F. One exemplary embodiment is illustrated in FIG. 6 and includes aforceps-like tool such as a rongeur tool 602 having a pair of jaws 604that are adapted to remove bone and create the desired notch. The notchmay also be created with files, chisels, rasps, grinders or other boneremoval instruments. These instruments may be manual or power tools,such as a drill, burr or saw. FIGS. 8C-8F illustrate other embodimentsof bone removal tools which may be used to create various features inthe sacrum. In FIG. 8C, a crescent shaped cutting edge 824 is attachedto a shaft 822 having a handle 820 for a surgeon to grasp. The crescentshaped cutting edge 824 is used to rongeur bone such as creating anundercut notch in a sacral crest which may be difficult for currentlycommercially available curettes. The cutting motion is a combination ofboth lateral and rotational movements as indicated by the arrows inFIGS. 8C-8D. The embodiment of FIG. 8C facilitates bone removal sincethe cutting edge 824 is offset from the shaft 822 of the tool, allowingdeeper penetration into the bone as the undercut is created andpreservation of midline tissues and structures. For example, it may bebeneficial to create a notch, while maintaining the supraspinousligament along the midline. An instrument that laterally accesses thebone removal site (such as those described with a cutting surface offsetfrom the shaft) will facilitate bone removal while maintaining themidline structures. An adjustment mechanism such as a set screw 826 maybe used to change the handle 820 orientation to facilitate use. FIG. 8Dis an enlarged view of the cutting edge 824 in FIG. 8C.

Another tool which may facilitate bone removal or creating attachmentfeatures in the sacrum is illustrated in FIGS. 8E-8F. In FIG. 8E, acurette tool has a knurled cutting surface 836 coupled to and offsetfrom a shaft 834 having a handle 830 for grasping. An adjustmentmechanism 832 such as a set screw allows adjustment of the handleposition relative to the cutting edge 836. The knurled curette tool mayalso be used to debride bone for creating notches such as undercuts inthe sacrum. A sharp tip or cutting edge 838 on the tip may be used tohelp the tool penetrate tissue. The knurled surface removes bone byabrading it away, similar to a file.

FIGS. 7A-7B illustrate an attachment method similar to that previouslydescribed with respect to FIGS. 5A-5F, with the major difference beingthat instead of creating a notch in a sacral crest, spinous process ortubercle of the sacrum, in this embodiment, an aperture is created inthese regions of the sacrum. In FIG. 7A, an aperture 708 is created in acrest or spinous process or tubercle of the sacrum, here the aperture isa circular hole. A constraint device 702 having a tether 706 and acompliance member 704 may be coupled with the sacrum S by passing atleast a portion of the tether structure 706 through the aperture 708.FIG. 7B shows a side view of the aperture 708 in the sacrum S. Theaperture 708 may be drilled into the sacrum S or a punch tool may beused to create the hole. For example, in FIG. 8A, a forceps-like tool802 similar to the rongeur tool of FIG. 6 has a pair of jaws 804 thatare adapted to create the aperture. In the embodiment of FIG. 8A, thepunch creates a round hole, although other shapes may also be created.FIG. 8B illustrates the forceps-like punch tool 802. The punch tool mayinclude a handle 806 for grasping in a surgeon's hand and the jaws 804which punch the aperture. A double-action mechanism 808 creates greaterleverage in use, thereby providing higher punch forces than asingle-action mechanism would. The punching jaws 804 in this embodimentcould easily be substituted with the notching jaws in FIG. 6 and thetools illustrated in FIGS. 8C-8F may also be used to create theaperture.

In some embodiments where an aperture or notch has been created in aportion of the sacrum, it may be advantageous to line the aperture ornotch with a protective liner element such as a bearing, liner, ferruleor grommet type of device. The liner is advantageous since it preventsdirect contact between the tether and the bone thereby minimizing wearand tear on the bone or tether. Furthermore, a liner can help to moreevenly distribute forces from the tether across the aperture or notch.FIGS. 18A-18B illustrate one embodiment of a liner. In FIG. 18A,aperture 1804 extends through a crest 1802 of the sacrum S. A two-pieceliner is inserted into the aperture 1804. Both halves 1806 and 1808 maythen be coupled together by press fitting, snap fitting, threading,bonding, welding, ratcheting mechanism, etc. to form a smooth channel1810 through the sacral aperture 1804. The liner may have flanged endsto conform with the outer surface of the aperture 1804 and the liner mayalso be malleable in order to conform with the anatomy. The liner may befabricated from many different metals such as stainless steel ortitanium, as well as many different polymers. Surface treatments such asthose disclosed in this specification may also be used to enhanceosseo-integration of the liner with the bone. Additionally, the channel1810 may be coated with various materials to provide desiredcharacteristics for the tether, for example a Teflon lining may be usedto provide a lubricious surface. The corners/edges of the liner mayfurther be designed and processed to reduce friction at the points atwhich the tether exits the liner. FIG. 18B illustrates a perspectiveview of the assembled liner.

FIG. 19 illustrates an alternative embodiment of a liner or grommet. InFIG. 19 , the grommet includes a main body section 1906, here acylindrical body that is sized to fit the aperture in the sacrum. Themain body 1906 also has a central channel or passage 1904 that passesthrough the body and accommodates the tether of the constraint device.In this embodiment, the grommet also has a number of fingers 1902 whichextend from either the proximal or distal end of the grommet. Thesefingers are fabricated from a superelastic or shape memory alloy such asNitinol. Therefore, the grommet may be placed in a sleeve forintroduction through the sacral aperture. Once the grommet is positionedin the sacral aperture, the sleeve may be retracted and the fingerseither self-expand around the edges of the aperture or self-expand uponreaching a desired temperature (e.g. above body temperature). Thus, thefingers 1902 form flanges which lock the grommet into the sacralaperture. Any of the coatings or surface treatments described herein mayalso be used in conjunction with this liner.

FIG. 20A illustrates a threaded liner 2002 that may be used to lineapertures and notches created in portions of the sacrum. In FIG. 20A, acylindrical tube having threads 2006 and a central channel 2004 may bethreaded into the sacral aperture. The liner may be threaded into thesacral aperture and the threads allow more secure anchoring of the liner2002 with the sacrum than other embodiments. FIG. 20B illustrates theuse of the threaded liner from FIG. 20A. In FIG. 20B, a constraintdevice has an upper tether 2010 coupled with an upper spinous process SPand a lower tether 2012 passes through the central channel 2004 of liner2002 that has been threaded into an aperture in the sacrum S. The liner2002 may have any of the surface treatments or coating described herein.

In addition to lining the internal surfaces of sacral apertures, theexternal surfaces of the aperture may also be reinforced. In FIG. 21 ,an aperture is created in a sacral crest or a spinous process ortubercle of the sacrum, and the tether structure 2104 of the constraintdevice is passed through the aperture. In this embodiment, one or bothof the external surfaces of the sacral aperture are reinforced so thatloads are not entirely borne by the bone. Here, a washer 2106 having acentral opening 2102 is positioned adjacent the sacral aperture. Thewasher 2106 may be positioned in a groove cut into the bone similar tothe embodiment described in FIGS. 4A-4C above so that it is flush withthe outer surface, or the washer 2106 may be secured to the bone forminga raised region. The washer may be sutured, strapped, fixtured, buckled,tied, twisted, ratcheted or bonded to the bone. In this embodiment, thewasher 2106 is held in position using a wire 2110 which passes throughan aperture 2108 in the washer and the other end of the wire may besecured to the bone. External reinforcement may also be combined withinternal lining. The washer 2106 may also include any of the surfacetreatments or coatings described in this application.

FIGS. 9A-9E illustrate a crossbar engaged with the sacrum as an anchormember for constraint device attachment. In FIG. 9A, a crossbar 902 iscoupled with the sacrum S transverse to the midline of the spinalsegment. This allows fixtures such as screws which penetrate the wingsor ala of the sacrum to avoid the spinal canal and also allows thefixtures to be placed deeper into the bone for improved purchase. Thecrossbar 902 may have a fixed length that matches the sacral anatomy orthe crossbar may have a sliding, telescoping or otherwise adjustablelength. Also, the crossbar may be malleable so that it may be formed tomatch the surface contours of the sacrum. Fixtures such as screws 906may be used to threadably attach the crossbar to the sacrum. In thisembodiment, four screws 906 are used to secure the crossbar with thebone. Other means for attaching the anchor member to the bone may alsobe used in conjunction with screws or by themselves, such as using bonecement, dowel pins, etc. In FIG. 9A, the crossbar 902 has an attachmentfeature 904 for engaging and holding the constraint device. Here, theattachment feature comprises two slots for receiving and holding aportion of the constraint device, such as the tether. The attachmentfeature could include other options such as passages, hooks or retentionfeatures such as clamps, some of which are disclosed below.

In FIG. 9B a constraint device comprises a tether 908 coupled with twocompliance members 910. A lower portion of the tether 908 is advancedthrough an aperture 916 in a tube 914 coupled with the crossbar 902.Apertures 918 on either end of the crossbar 902 are used to receive afastener such as a screw 912 so that the crossbar 902 is secured to thesacrum. In FIG. 9C, a post 920 is disposed on the crossbar 902 and thelower portion of the tether may then be wrapped around the post.Additional details involving the use of a post coupled either directlywith the sacrum or coupled to a crossbar are described below in otherexemplary embodiments.

In FIG. 9D, the tether structure 908 has two free ends which may bethreaded through apertures 922 in the crossbar 902. The ends may then besecured by knotting the free ends, pinning them in place, locking them,or by using other methods well known in the art. Other applicablelocking mechanisms are disclosed below. In FIG. 9D, the crossbar 902 maybe secured to the sacrum with screws 912 that pass through apertures 918on either end of the crossbar. FIG. 9E illustrates another embodiment ofattachment of the constraint device to the sacrum. In FIG. 9E, acrossbar 902 is fixed to the sacrum S with pins or screws 930 on eitherend of the crossbar 902. The crossbar 902 has an elongated, ellipticallyshaped aperture 932 which may receive a portion of the constraintdevice. In this embodiment, the constraint device comprises an uppertether 924 which is disposed around a superior surface of a superiorspinous process SP. The upper tether 924 has a fixed end and a free end926 which is engaged with a first compliance member 934. The free end926 may be pulled through a locking mechanism on the compliance member934, thereby adjusting length or tension in the constraint device. Thefixed end is fixedly coupled with the second compliance member in thisembodiment, although it also could be adjustable. A lower portion of thetether 936 also has a fixed end and a free end 928. The fixed end may befixed with the first compliance member or it may be adjustable. The freeend is similar to the upper tether free end 926, and it may beadjustably coupled with the second compliance member. The lower portionof the tether is threaded through the aperture 932, thereby securing thetether and constraint device to the sacrum S. While this method ofattaching the constraint device to the sacrum is relatively easy toperform, in some situations the method may result in twisting of theconstraint device which causes asymmetry in the device along withunbalanced forces applied to the spinal segment.

FIG. 10 illustrates the use of a screw as the sacral anchor member. InFIG. 10 , an anchor screw 1002 is threadably engaged with the sacrum. Inthis exemplary embodiment, the screw 1002 may be screwed into a crest ofthe sacrum axially in the caudal direction. This allows the screw to bemuch longer than if it were threaded perpendicularly in an anteriordirection into the bone. A screw directed in the anterior directioncould penetrate either the spinal canal or the anterior cortex of thesacrum and the colon. By directing the screw axially in the caudaldirection permits a longer screw to be used without fear of penetratingthe regions listed above, and allows a longer portion of the screwthreads to be engaged with the bone, thereby providing greater purchase.A tether structure 1004 may be threaded through an aperture 1006,eyelet, hook or other attachment feature on the screw. The orientationof the screw also helps distribute loads primarily in the direction ofthe screw's longitudinal axis which is desirable since it results inless bending moments exerted on the screw which can cause screw fatigue,loosening and “toggling,” all which are commonly associated with bonescrews. In alternative embodiments, the screw may be oriented in acranial direction.

Instead of using a screw as in FIG. 10 , a pin may be used to helpcreate or enhance the sacral attachment region. In FIG. 22 , a pin 2202that is inserted in the cranial direction into a crest, spinous processor tubercle 2204 of the sacrum S. A portion of the pin is embedded inthe bone while a portion is left unembedded. The unembedded portion ofthe pin forms an overhang and the tether 2206 of the constraint deviceis captured between the overhang and the sacrum. Thus the tether iscaptured on an inferior side of the sacral crest. Here, a single pin isused, although more than one pin may be used if required. In otherembodiments, the pin may include a flanged rim, boss or land thatprevents the pin from further penetration into the bone. The flange maybe fixed or it may be adjustable so that penetration depth may bevaried. Additionally, the pin may have threads on the shaft in order tofacilitate implantation into the bone and enhance fixation. Still otherembodiments of the pin may have an arcuate section such that as the pinis advanced into the bone, the curve forces the pin in a direction awayfrom and prevents penetration of the pin into the sacral canal.

Lateral retaining bars may also be used to retain the tether to thesacrum. In FIG. 23A, a rectangular shaped retaining bar 2306 islaterally positioned in a crest, spinous process or tubercle 2302 of thesacrum S. In this embodiment, a single bar extends from one side of thecrest 2302 to the other side and a portion of the retaining bar 2306extends past each side of the crest 2303, forming a flange. Tether 2304may then be held in position by the flanged portion of the retaining bar2306. In FIG. 23A, the retaining bar may be a single bar pushed throughthe crest 2302 or two separate bars may be pressed into the crest 2302from opposite sides. Additionally, the retaining bar 2306 may be presseddirectly into the bone, or a channel may be drilled or otherwise cutinto the bone before receipt of the bar. In FIG. 23B, a single retainingbar 2308 is pushed through the crest 2302. One end of the retaining barhas a T-shaped end 2312 which serves as a stop to prevent the bar frompenetrating too deeply into the bone and also creates the flanged regionfor retaining the tether 2304. A pin, spline or other elongate element2310 may then be placed through the opposite end of the bar in order tolock the bar into position and to create a second flanged region forretaining the tether 2304. Additionally, in other embodiments, theretaining bar ends may be curved to help enhance retention of the tetherby forming hook-like regions as well as preventing sharp or long endsfrom protruding as well as producing a profile that may approximate thesacral surface more closely.

Retaining clips are also useful way of engaging a tether with thesacrum, as seen in FIGS. 24A-24C. In FIG. 24A a retaining clip 2402 hastwo legs 2408 and a pair of flanges 2404 or wings. Sharpened fingers2406 help engage the clip with bone without requiring deep penetrationinto the bone. The fingers 2406 may comprise hooks, barbs or teeth toimprove purchase on the sacral surface. In use, the clip may bestretched open and then allowed to spring back and close around aspinous process, tubercle or crest of the sacrum S. Alternatively, theclip 2402 may be pressed and deformed against these regions. In eithercase, the clip is then securely attached to the raised area of thesacrum S and a portion of the tether is captured under the flanges 2404and secured to the sacrum S. FIG. 24B illustrates a side view of aconstraint device having an upper tether portion 2410 coupled with asuperior spinous process SP and a lower tether portion 2414 capturedbetween the wings or flanges 2404 of the retaining clip 2402 and thesacrum S. Compliance members 2412 join the upper and lower tethers. FIG.24C illustrates a back view of the FIG. 24B. Any of theosseo-integration surface treatments or coatings disclosed herein mayalso be used with retaining clip 2402 to help engagement of the clipwith the bone.

Additional embodiments of retaining clips are illustrated in FIGS.25A-25E. In FIG. 25A, the clip may be attached to a crest, spinousprocess or tubercle of a sacrum in the same manner as previouslydescribed with respect to FIGS. 24A-24C. Retaining clip 2502 has twolegs 2504, 2506 which may spring into engagement with the sacrum S orwhich may be deformed into engagement with the sacrum S. Fingers 2508which may take the same form as the fingers 2406 help the clip engagethe bone. The retaining clip has an aperture 2510 extending through bothlegs 2504, 2506. The tether may be passed through this aperture, therebysecuring the tether to the clip and sacrum. FIG. 25B illustrates analternative embodiment of a retaining clip similar to that of FIG. 25A,except this embodiment does not have aperture 2510 and instead has anoverhang 2516 which forms a lip for retaining the tether. FIG. 25Cillustrates a side view of the retaining clip in FIG. 25B when clippedto the sacrum S and how overhang 2516 retains tether 2518. FIG. 25D is aback view of FIG. 25C. FIG. 25E is a side view showing the retainingclip of FIG. 25A when clipped to the sacrum S. Tether 2518 is secured tothe clip 2504 by passing through aperture 2510. Any of these retainingclips may also include the osseo-integration coatings and surfacetreatments described herein to help engagement of the clip with thebone.

Other clip-on anchors may include wire-like staples such as thoseillustrated in FIGS. 26A-26D. In FIG. 26A, a wire-like clip or staple2602 is attached to a spinous process, tubercle or crest SP of thesacrum. The clip may be applied with a staple-like gun or the device maybe applied manually by a surgeon. A tether may be coupled with thewire-like clip 2602 by passing it through a looped portion 2606 in theclip. FIG. 26B is an enlarged view of FIG. 26A. The staple-type anchormay include a series of contact points such as seen in FIG. 26C. Here,the retaining clip 2610 is applied with a staple-type gun or manually bythe surgeon and a hook or overhang region 2612 allows the clip to retainthe tether 2614. FIG. 26D represents a cross-section of FIG. 26C takenalong line A-A.

Any of the retaining clips or other devices disclosed herein may furthercomprise a carabiner-style gate for receiving and retaining a part ofthe constraint device. FIG. 27 illustrates one such embodiment. In FIG.27 , a gate 2702 is movable from a closed position to an open position.In the open position, a portion of a constraint device such as a tethermay be passed into the receiver 2704. Once the gate is closed, thetether is captured by the clip. The gate may be spring loaded so that itis biased into the closed position.

Other mechanisms for retaining a tether to the sacrum include the use ofa retaining plate that is coupled to adjacent crests, spinous processesor tubercles of the sacrum. For example, in FIG. 28A, a retaining plateincludes an elongate middle section 2802 having an enlarged fork-likehead and tail sections 2804, 2806. Each fork-like section is U-shapedwith the arms of the U forming a receptacle in which a portion of thesacrum may rest. Apertures 2808 on both head and tail sections allow apin 2812 or other fixture to secure the retaining plate to the bone.FIG. 28B illustrates a side-view of a retaining plate 2802 coupled tothe sacrum and retaining a portion of a constraint device. A constraintdevice has an upper tether portion 2814 coupled with a superior spinousprocess SP and a lower tether portion 2818 disposed and retained underthe retaining plate 2802. A compliance member 2816 is coupled betweenthe upper and lower tether portions. The retaining plate 2802 hasopposite ends pinned to raised regions 2820 of the sacrum. The raisedregions 2820 may be a crest, spinous process or tubercle. FIG. 28Cillustrates a dorsal view of FIG. 28C.

In some situations, it is preferable to use the existing naturalfeatures of the sacrum for tether anchoring rather than modifying thebone with notching or other bone removal procedures or screwing,stapling or pinning anchors into the bone. Screws in particular havemany known limitations and co-morbidities such as invasiveness, bloodloss, toggling or loosening of the screw, fatigue or fracture, etc.Therefore, anchors which directly appose bone without resection may beadvantageous. Hooks for example are desirable since they are lessinvasive than screws or nails and even anchors that do penetrate bonewith teeth or nails may still be preferable to screws if they requireless bone resection and penetration, or are easier to implant anddistribute loads more evenly or provide other beneficial features. FIG.29 illustrates the use of hooks engaged with neural foramina of thesacrum. FIG. 29 illustrates neural foramina 2912 in the sacrum S. Neuralforamina 2912 are often lateral of the spinal segment midline, thereforetwo foramina on opposite sides of the midline may be joined with atransverse retaining device. In FIG. 29 , a retaining device includes apair of hooks 2914 engaged in neural foramina 2908 of the sacrum S. Thehooks 2914 are coupled together with a transverse member 2916 such as abar, wire, tether or other cross member. The retaining device is used tosecure a portion of a constraint device to the sacrum S and may straddlethe paraspinal muscles (see FIG. 31 ). Constraint device 2910 includesan upper tether portion 2902 coupled with a superior spinous process SPand a lower tether portion 2906 is looped around and captured by thecross member 2916 of the retaining device 2910. A pair of compliancemembers 2904 couple the upper and lower tether portions 2902 and 2906together.

FIGS. 30A-30D illustrate the anatomy of a neural foramen and variousconfigurations of foramenal hooks. FIG. 30A illustrates a cross sectionof a sacral foramen 3012. The foramen is an aperture between an upper(cranial direction 3002) and lower (caudal direction 3004) portion ofthe sacrum. A nerve 3006 coupled with the nerve root running in thespinal canal, near the dorsal surface of the spinal canal 3008 may exitthe foramen and may run adjacent the dorsal sacral surface 3010. Becauseof the presence of the nerve 3006, care must be used to avoid injuringthe nerve when hooks are placed in the foramen. FIGS. 30B-30D illustratevarious configurations of neural foramina hooks which may be used toengage the foramen. In FIG. 30B, a hook 3020 has a single arm which ispositioned in the foramen 3012 and cranially-oriented such that the hookis retained when under tension. In FIG. 30C, hook 3022 has two arms, onearm 3022 is positioned in the foramen 3012 and cranially oriented and asecond arm 3024 which is positioned in the foramen 3012 and caudallyoriented, thus the hook 3020 is retained in the foramen. FIG. 30Dillustrates still another foramenal hook embodiment, this embodimentincludes a hook 3024 having a claw configuration with the clawpositioned in adjacent foramen so that the claw grasps the bone inbetween the two foramina. Thus, in FIG. 30D, the hook remains engagedwith the sacrum.

The foramina hooks previously described have many advantages, but mustbe used with caution in order to avoid pinching the nerves in the area.Therefore, in some situations, it still may be advantageous to usehooks, but to engage them with another portion of the sacrum thatminimizes or avoids pinching nerves. FIG. 31 illustrates a retainingdevice having a pair of hooks 3108 that are engaged with a lateral edge3110 of the sacrum S. A transverse connector such as bar, rod, wire,tether or other cross member 3112 joins the two hooks 3108 together. Alower tether portion 3106 of the constraint device may then be loopedaround the cross member 3112 and secured to the sacrum S. The remainderof the constraint device includes an upper tether portion 3102 engagedwith a superior spinous process SP and a pair of compliance members 3104joining the upper and lower tether portions together. In addition toavoiding the nerves in the foramina, this embodiment also allows theretaining device to straddle the paraspinal muscles P thereby causingminimal disruption to the muscles. Any of the hook embodimentspreviously described may be used to hold the retaining device to theedges of the sacrum.

An anchor member may be attached to the sacrum without screws as seen inFIGS. 11A-11D. In FIG. 11A, an anchor plate 1106 having a plurality ofnail-like protrusions 1108 is engaged with the sacrum S. In thisembodiment, the nails are inclined relative to the outer surface of theplate 1106 to help resist applied loads from the constraint device whichincludes an upper tether portion 1116 coupled with a superior spinousprocess SP and a lower tether portion 1112 coupled with the anchor plate1106 via an aperture 1110. In other embodiments, the anchor plate 1106may have a plurality of apertures through which individual nails may bepassed. Using a plurality of nails distributed along the length of theanchor allows loads to be distributed over more points therefore shorternails may be used, avoiding the need for deep penetrating nails.Preferably, the nails only penetrate the cortical bone on the dorsalsurface of the sacrum, minimizing the risk of neural injury, butpenetration may be varied as required. A compliance member 1114 isdisposed between the upper 1116 and lower 1112 portions of the tether.In this exemplary embodiment, the anchor plate 1106 may be insertedthrough a minimally invasive incision through the skin SK and thenhammered into position with a custom placement tool 1104 and mallet1102. In this embodiment, the aperture 1110 allows coupling of theanchor plate 1106 with the tether, although other attachment featuressuch as a hook may be used. One of the advantages of this type of anchoris that there is greater flexibility in attachment of the tetherstructure in the cranial or caudal direction. Attaching the tetherstructure to native anatomy such as a spinous process or a notch in thesacrum is likely to be limited to specific areas of the sacrum. Usingthe anchor plate allows the attachment region to be moved in the cranialor caudal direction which will help accommodate the compliance memberand other portions of the tether structure.

An alternative embodiment of the anchor plate 1106 may be seen in FIG.11B. In FIG. 11B anchor plate 1106 a comprises a hook 1118 for engagingthe tether structure and instead of nails, a plurality of hooks, clawsor barbs 1108 a are used to secure the anchor plate 1106 a to the sacrumS. FIG. 11C shows a constraint device having an upper tether portion1116 coupled with a superior spinous process SP and a lower tetherportion 1112 coupled with the hook 1118 on anchor plate 1106 a. A freeend 1120 of the upper tether 1116 and a free end 1120 of the lowertether 1112 pass through a locking mechanism 1122 adjacent eachcompliance member 1114. Thus, length or tension in the tethers may beadjusted. Additional details on the locking mechanism are disclosedelsewhere in this application. FIG. 11D shows a side view of theconstraint device coupled with a spinous process SP and the anchor plate1106 a.

Additionally, the surfaces of anchor plate 1106 that engage the sacrum Smay be treated to create a surface that promotes osseointegration of theplate with the bone. Exemplary treatments include beading as well ashydroxyapatite and titanium coatings. These surface treatments may beapplied to any of the implants disclosed in this specification.

Referring now to FIG. 12 , pins or screws 1208 are placed in the sacrumS and help to secure a constraint device to the bone. In FIG. 12 , aconstraint device includes an upper tether portion 1202 that is disposedover a superior spinous process and a lower tether portion 1206 isdisposed around two screws 1208 threaded into the sacrum S. Twocompliance members 1204 join the upper and lower tether portionstogether. Additionally, a free end 1210 of the upper tether portion 1202is received by one of the compliance members 1204 and allows adjustmentof the tension or length in the constraint device.

In FIG. 13 a crossbar 1302 is anchored to the sacrum S with two screws1304 on either end of the crossbar 1302. In this embodiment, theconstraint device includes an upper tether 1306 looped around a superiorspinous process SSP with a free end 1308 engaged with a lockingmechanism 1312 on the compliance member 1310. The locking mechanismallows the length and tension of the constraint device to be adjusted toaccommodate different patients. Various locking mechanisms are disclosedin U.S. patent application Ser. No. 12/479,016 (Attorney Docket No.026398-000710US) and PCT Publication No. WO 2009/149407 (Attorney DocketNo. 026398-000810US), the entire contents of both are incorporatedherein by reference. Also in this embodiment, a lower portion ofcompliance member 1310 is coupled with a post or raised screw 1312disposed on the crossbar 1302. The raised screw 1312 is preferablylocated along the spinal segment midline, although it may be located offcenter if desired.

FIGS. 14A-14B illustrate embodiments of constraint devices anchored tothe sacrum S with a pin or screw. In FIG. 14A, a constraint deviceincludes an upper tether 1402 disposed at least partially around asuperior surface of a superior spinous process SP. One end of the uppertether 1402 is pre-attached to a first compliance member 1408 while theopposite end of tether 1402 is a free end 1404 that is received in alocking mechanism 1406 of a second compliance member 1410. The lockingmechanism 1406 allows length and/or tension in the constraint device tobe adjusted. Any of the locking mechanisms previously described abovemay be used in this embodiment. The first compliance member 1408 iscoupled to a loop 1412 in the tether structure and the loop 1412 issecured to a post or screw 1414 in the sacrum S. An optional adjustingmechanism 1416 may be included in the first compliance member 1408 orthe second compliance member 1410 in order to allow further adjustmentof length and/or tension in the constraint device. A second loop 1412 iscoupled with the second compliance member 1410 and a pin or screw 1414secures the second loop 1412 to the sacrum S.

The embodiment of FIG. 14B is similar to that in FIG. 14A with the majordifference being that the tether structure in FIG. 14B has two free endsthat may be adjusted. In FIG. 14B, the constraint device includes anupper tether 1402 disposed at least partially around a superior surfaceof a superior spinous process SP. Both free ends 1404, 1405 of the uppertether are received in a locking mechanism 1406 in each of the twocompliance members 1408, 1410. The locking mechanism 1406 may be any ofthose previously disclosed above. Similar to the embodiment of FIG. 14A,looped ends 1412 of the tether are coupled on one end to a compliancemember, either 1408, 1410 and secured at the other end to a pin or screw1414 in the sacrum S. In alternative embodiments the looped ends 1412may be coupled to one notch or aperture in the sacrum (similar to FIG.5E, 5F or 7A), or in still other embodiments the looped ends areindividually coupled to a notch or aperture, one on each side of thespinal segment midline.

The embodiments of FIGS. 15A-15E use a pin or post secured to a sacralcrossbar as an attachment point for the constraint device. In FIG. 15A,a crossbar 1522 is secured to the sacrum S with two screws 1512 oneither end of the crossbar. A central pin or post 1516 is coupled withthe crossbar 1522 and is used to attach a constraint device to thesacrum. The constraint device in FIG. 15A includes an upper tether 1502disposed around an upper surface of a superior spinous process SP. Afree end 1506 of the upper tether 1502 is received in a lockingmechanism 1504 of a first compliance member 1508 so that tension and/orlength may be adjusted. The opposite end of upper tether 1502 ispre-attached with a second compliance member 1508. A lower tether 1514is disposed around a central pin or post 1516 and includes a free end1510 that is also received in a locking mechanism 1520 of the secondcompliance member 1508 to allow additional adjustment of length and/ortension in the constraint device. The opposite end of the lower tether1514 is pre-attached with the first compliance member. FIG. 15Billustrates the central pin or post 1516 in greater detail. The post1516 comprises a central concave region 1520 disposed between twoenlarged shoulder or head regions 1518. This helps prevent the tether1514 from sliding off the post.

The embodiment of FIGS. 15C-15D is similar to the embodiment illustratedin FIGS. 15A-15B with the major difference being how the compliancedevice is coupled to a central post. As in FIG. 15A, a crossbar 1522 issecured to the sacrum S with two screws 1512 on either end of thecrossbar 1522. The constraint device in this embodiment generally takesthe same form as the device described with respect to FIG. 15A. In FIG.15C, central post 1513 has an aperture and the lower tether 1514 may beadvanced through this aperture and then secured to the compliancemembers 1508. FIG. 15D illustrates the central post 1513 in greaterdetail. The aperture may be elliptical or rectangular or any other shapethat may receive the tether structure 1514. An additional lower aperture1526 allows the central post 1513 to engage the crossbar 1522 and a setscrew 1530 may be used to secure the components together.

Various embodiments of posts or pins are illustrated in FIGS. 16A-16F.Any of these may be fabricated from metals such as titanium or stainlesssteel, or they may be made from other materials such as polymers orceramics. Additionally, any of these may be used in any of theembodiments disclosed above where the constraint device is eitherwrapped around a post, pin, or screw or threaded through an aperture inthe post, pin, or screw. These posts or pins may be secured directlyinto the sacrum or to an anchor member such as a crossbar that issecured to the sacrum. The posts/pins may be press fit into engagementwith the sacrum or the anchor member, or they may be coupled withanother fastener such as a screw that is threadably engaged with thebone or anchor. Referring now to FIG. 16A, a pin has a elongatecylindrically shaped body 1602 having a substantially constant diameter.A hex head 1604 is provided so that a tool may engage and rotate thepin. One of skill in the art will appreciate that other heads such as aslot, Phillips, Torx, etc. may easily be substituted for the hex head inthis embodiment as well as any of the screw, pin or post embodimentsdisclosed herein. In FIG. 16B, the pin has an elongate cylindrical body1606 also with a constant diameter, except that this embodiment alsoincludes an enlarged head/shoulder region 1608 that is coupled with thecylindrical body 1606 in order to prevent a tether from slipping off thepost. This embodiment also includes a hex head 1604. The embodiment ofFIG. 16C includes a concave central region 1610 disposed between twoenlarged head/shoulder regions 1612, one of which includes a hex head1604. Again, the enlarged regions help prevent slipping of a tether offthe post.

FIG. 16D illustrates yet another embodiment of a pin or post that may beused to help secure a constraint device either directly or indirectlywith a sacrum. In FIG. 16D, the central region of the pin is concave andit is surrounded on either side by an enlarged head/shoulder region1616. One end of the post includes a hex head 1604. The transition fromthe concave region to the enlarged head/shoulder region includes anannular flange 1618 on both ends and this feature is also helpful forpreventing slippage of the tether structure off the pin/post. In FIG.16E, the pin/post has a cylindrical body 1620 with a central slot 1622or aperture for receiving the tether structure. Often, one end of thetether is free so that it may be threaded through the slot and thensecured to the remainder of the constraint device with a lockingmechanism. Once the tether is fed through the slot, it is captured andcannot slip off the post. In this embodiment, the slot is elliptical inshape, although one of skill in the art will appreciate that other slotgeometries may also be used. FIG. 16F illustrates still anotherembodiment of a post/pin. In this embodiment, a central cylindricallyshaped region of constant diameter 1624 is surrounded on either side byan enlarged head/should region 1626. The transition from the centralcylindrical region 1624 to the enlarged head regions 1626 includes astep or flanged region 1628 on both ends.

As previously mentioned, any of the post/pin embodiments may be coupledwith a screw for direct engagement with the sacrum. FIGS. 17A-17Billustrate two exemplary embodiments of post/pins having threadedregions that may be threadably engaged with the sacrum. In FIG. 17A, ascrew 1702 includes a head region having a recessed step 1706 forholding a tether. A flange region 1708 prevents the tether from slippingoff the post. A hex head or other driver feature may be included on atop surface of the flange region. A tapered head region 1710 allowsbetter grip with cortical bone and a tapered thread 1712 increases thegrip of the screw with bone. The dual lead 1704 design is also desirablesince it allows faster and more efficient penetration of the screw intothe bone. FIG. 17B illustrates an alternative embodiment of a screw 1714that can be threaded into the sacrum. In FIG. 17B, flanged region 1708prevents slippage of the tether off the pin and provides an area for adriver feature such as a hex head. An increased thread pitch 1718, duallead 1704 and deeper thread 1720 permit more efficient cutting into thebone. This embodiment also includes a tapered head 1716. Exemplarydimensions of these screws may include an outer diameter from about 5 mmto about 8 mm, and more preferably from about 6.5 mm to about 7.5 mm,with a thread length ranging from about 20 mm to about 70 mm and morepreferably from about 35 mm to about 50 mm.

Many embodiments disclosed above relate to attachment of a constraintdevice to the upper sacrum. An alternative attachment location could bethe lower sacrum such as the coccyx or sacro-coccygeal junction. Forexample, in FIG. 32 , a constraint device has an upper tether portion3202 coupled to a superior spinous process SP and a long lower tetherportion 3208 that passes over the sacrum and is coupled to the coccyx3210. A pair of compliance members 3204 (only one illustrated in thisside view) join the upper and lower tether portions. This configurationis desirable since the lower tether portion cannot slip off.Additionally, the tethers are oriented parallel to the spinal midlineand medially oriented close to the midline which are preferred positionsfor a flexion restricting device. Various embodiments of constraintdevices which may be coupled with the lower sacrum are illustrated inFIGS. 33A-33B.

For example, the constraint device in FIG. 33A includes an extra longlower tether portion 3306 that can accommodate the additional distancerequired to be looped around the coccyx. The upper tether portion 3302is shorter than the lower portion, but sized to be coupled with an upperspinous process. A pair of compliance members 3304 join the upper andlower tether portions. In FIG. 33B, the lower tether portion 3310 is asingle tether having a looped end 3312 for looping around the coccyx.Additionally, a connector or shackle 3308 is attached to an upper partof the lower tether portion 3310 and allows the lower tether portion tobe joined with a constraint device such as that illustrated in FIG. 2A.

The constraint device may be coupled with the coccyx in severaldifferent surgical procedures such as those illustrated in FIGS.34A-34B. For example, in FIG. 34A, an upper tether portion 3402 of theconstraint device is coupled with a superior spinous process SP. Anextra long lower tether portion 3406 is then advanced subcutaneouslycaudally toward the coccyx and then looped around the coccyx 3410. Thefree end 3408 of the lower tether portion is then advanced in thecranial direction and coupled with a compliance member 3404 which isalso coupled with the upper tether portion 3402. In FIG. 34B, the uppertether portion 3402 is coupled with an upper spinous process SP. Asecond incision may be made and a lower tether portion 3414 is loopedaround the coccyx 3410 and the free end 3416 is advanced subcutaneouslyin the cranial direction wherein it will be connected to a tail 3412 onthe compliance member 3404. Orthopedic cables, wires, sutures, bandpassing instruments are well known in the art and may be used tofacilitate movement of the tethers

While the above is a complete description of the preferred embodimentsof the invention, various alternatives, modifications, and equivalentsmay be used. Therefore, the above description should not be taken aslimiting the scope of the invention which is defined by the appendedclaims.

What is claimed is:
 1. A method for restricting flexion of a spinalsegment of a patient, said method comprising: providing a constraintdevice having a tether structure, the tether structure comprising anupper portion and a lower portion, the tether structure configured torestrict flexion of the spinal segment while allowing substantiallyunrestricted spinal extension; coupling the upper portion of the tetherstructure to a spinous process of a superior vertebra in the spinalsegment; anchoring an anchor element to a sacrum of the patient inferiorto the superior vertebra, wherein the anchor element comprises a platehaving a plurality of protrusions, the plurality of protrusionsconfigured to penetrate the sacrum, wherein anchoring the anchor elementcomprises anchoring the anchor element to the sacrum without usingscrews, and wherein anchoring the plurality of protrusions onlypenetrates cortical bone on a dorsal surface of the sacrum; and couplingthe lower portion of the tether structure to an attachment feature onthe anchor element.